How dangerous is climate change? A 250 mn-year-old event may hold clues

Some 252 million years ago, Earth almost died.In the oceans, 96 percent of all species became extinct. It’s harder to determine how many terrestrial species vanished, but the loss was comparable.

This mass extinction, at the end of the Permian Period, was the worst in the planet’s history, and it happened over a few thousand years at most — the blink of a geological eye.

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On Thursday, a team of scientists offered a detailed accounting of how marine life was wiped out during the Permian-Triassic mass extinction. Global warming robbed the oceans of oxygen, they say, putting many species under so much stress that they died off.

And we may be repeating the process, the scientists warn. If so, then climate change is “solidly in the category of a catastrophic extinction event," said Curtis Deutsch, an earth scientist at the University of Washington and co-author of the new study, published in the journal Science.

Researchers have long known the general outlines of Permian-Triassic cataclysm. Just before the extinctions, volcanoes in what is now Siberia erupted on a tremendous scale. The magma and lava that they belched forth produced huge amounts of carbon dioxide.

Once in the atmosphere, the gas trapped heat. Researchers estimate that the surface of the ocean warmed by about 18 degrees Fahrenheit. Some researchers argue that the heat alone killed off many species.

Others believe that the warmth reduced oxygen in the ocean, asphyxiating the species living there. Rocks from the mass extinction appear to have formed when at least some of the ocean was lacking oxygen.

In previous research, Deutsch has explored how living animals adapt to temperature and oxygen levels in the seas. Animals with a fast metabolism need a lot of oxygen, for example, and so they can’t live in parts of the ocean where oxygen falls below a certain threshold.

Warm water makes the challenge even more difficult. Warmer water can’t hold as much dissolved oxygen as cold water. Even worse, warm water can also increase an animal’s metabolism, meaning it requires more oxygen just to stay alive.

Cod, for example, are not found below a latitude running roughly from New England to Spain. South of that line, warmth and low oxygen are just too great for the species.

Deutsch and Justin Penn, a graduate student, recreated the world at the end of the Permian Period with a large-scale computer simulation, complete with a heat-trapping atmosphere and a circulating ocean.

As the Siberian volcanoes flooded the virtual atmosphere with carbon dioxide, the atmosphere warmed. The ocean warmed, too — and according to the model, it began losing oxygen.

Some parts lost more than others. On the surface, for example, fresh oxygen was produced by photosynthetic algae. But as the ocean warmed, its circulatory currents also slowed, the model demonstrated.

Oxygen-poor water settled to the bottom of the oceans, and before long, the deep was gasping.

Rising temperatures and plunging oxygen must have made huge swaths of the oceans uninhabitable. Some species survived here and there. But most disappeared completely.

“Everything was losing a lot of habitat, creating the risk of extinction," said Deutsch. “But the risk was actually higher in places that were cold. That was a bit surprising."

You might expect that animals near the Equator would be at a greater risk, because the water was warm to begin with. But Deutsch’s model suggested a very different kind of apocalypse.

Animals in oxygen-rich cold water could not handle the sudden drop, while those in tropical waters were already adapted to poor oxygen. And the cold-water species could not find refuge elsewhere.

To test their simulation, the researchers teamed up with Jonathan Payne and Erik Sperling, paleontologists at Stanford University.

They dug into a huge online database of fossils to chart the risks of extinction at different latitudes during the catastrophe.

When they were done with their analysis, they sent their graph to Seattle. Deutsch and Penn compared it to the prediction from their computer model.

They matched. “This was the most exciting moment of my scientific life," said Deutsch.

Michael Benton, a paleontologist at the University of Bristol in England, who was not involved in the study, said that it resolved the roles of heat and oxygen as causes of the mass extinction.